The "Send Email" button indicates the step in the workflow where a mailing should be sent out of Delivra. Automation campaigns (drip campaigns) MUST either start with the "Send an Email" or "Send SMS" message.
As noted, a drip campaign MUST start with either Send Email or Send SMS.
For the first email, simply click the pencil on the workflow step and choose either a Regular or Triggered email to start off your drip campaign.
For any subsequent Send Email steps, you must choose the Automated mailing type.